There is a general tendency to replace the traditional incandescent lamps by other types of light sources, such as LEDs and gas discharge lamps. LEDs and gas discharge lamps have, with respect to each other, some advantages and disadvantages, and a designer may choose to use either an LED or a gas discharge lamp, depending on his design considerations.
A light source, be it an incandescent lamp, an LED or a gas discharge lamp, is designed for nominal operation at a nominal lamp voltage and a nominal lamp current, resulting in a nominal lamp power and a nominal light output. If, in a certain situation, a user wishes to have more light, he may replace the current lamp by a more powerful lamp, or by a lamp of a different type having a higher light output. Conversely, if a user wishes to have less light, he may replace a lamp by another lamp having a lower light output. However, this is very cumbersome, so there is a general desire to be able to dim a lamp, i.e. to drive a lamp with a power below its nominal power such that the light output is less than the nominal light output.
The present invention relates particularly to the field of driving a gas discharge lamp at reduced power, i.e. in a dimmed state.
A gas discharge lamp has a negative resistance characteristic, and therefore a ballast device is needed for driving the lamp. Although, in principle, it is possible to drive a gas discharge lamp with DC current, an electronic ballast typically provides a high frequency alternating current. Dimming can for instance be achieved by reducing the magnitude of the lamp current, or by switching the lamp on and off at a certain duty cycle.
Several problems and disadvantages are associated with the different mechanisms for dimming a gas discharge lamp, depending among others on the specific use, especially if it is desirable that the lamp is dimmed to a very low level of less than 1% of the nominal light output. A particular light generating device to which the present invention relates is a so-called wake-up light, which is a device which, triggered for instance by a clock, gradually increases its light output from zero to maximum. One of the problems for such an application is associated with ignition. For its ignition, a gas discharge lamp requires a relatively high voltage. As a result, if the lamp is to be ignited in the dimmed condition with a light output close to zero, the lamp may produce a light flash on ignition and then reduce its light output to the desired dim level. Such a light flash is undesirable.
A further problem is that it is very difficult to maintain lamp stability at a very low dim level.
In the case of gas discharge lamps having filament electrodes, the electrodes need to be supplied by an electrode heating current in order to keep the electrodes at an optimum operative temperature. However, in typical electronic ballasts, the filaments are only heated in the ignition phase, and during dimming the temperature of the filaments may become too low. Thus, it may be necessary to provide a separate electrode heating circuit, but such circuits tend to be complex and relatively expensive.
In a linear gas discharge lamp, the electrodes are arranged at opposite ends of a longitudinal lamp tube. The traditional TL lamp is an example of such a linear lamp. A disadvantage of such a lamp is that the lamp sockets for receiving the lamp terminals in a luminaire must be arranged at a relatively large distance from each other. As an alternative, so-called compact gas discharge lamps have been developed, where the lamp tube can be considered as being folded so that the lamp comprises an even number of tube segments arranged parallel next to each other, while the lamp ends with the lamp electrodes are located next to each other at the same longitudinal end of the lamp. Such a lamp can easily be mounted on a lamp base having a screw cap for screwing the lamp into a standard screw fitting, for instance in order to replace traditional incandescent lamps. In such a lamp type, in the case of application as a wake-up light with very low dim levels, an instability problem may occur in that the lamp, during the first stage of the wake-up sequence, will only emit light from lamp portions close to the electrodes, which portions relatively slowly grow away from the electrodes towards the other end of the lamp, while the intermediate tube segments do not emit light.
The present invention specifically aims to provide a solution to this problem.